Nowadays the tight condition for energy resources has increased the demand for drilling, transportation and storage of a crude oil and natural gas including hydrogen sulfide. Thus, materials to be used for these industries are required to provide a higher strength than in the past in order to meet the requirement of deeper drilling, more efficient transportation and the reduction of drilling cost by using a thinner pipe.
More particularly, typical steel pipes conventionally used have a yield stress (YS) of 80 to 95 ksi class. On the other hand, recently a steel pipe of 110 ksi class has been used, and the market is demanding a steel pipe of 125 ksi or above.
Among the steels with high sulfide stress cracking resistance (hereinafter referred to as SSC resistance), the following are known ; (a) a steel with martensitic microstructure of 80 to 90% or more, (b) a steel which is free from coarse carbides, (c) a clean steel containing less non-metallic inclusions, (d) a steel tempered at a high temperature, (e) a steel with fine grain sizes, (f) a steel having a high yield stress ratio, (g) a steel containing low Mn-low P-low S, (h) a steel containing abundant insoluble nitride, and (i) a steel added with zirconium.
There are various processes for producing a high strength low alloy steel having excellent SSC resistance. As a typical process, there is a rapid heating method, as disclosed in Japanese Laid-Open Patent Publications S54-117311 and S61-9519 and a short time tempering method, as disclosed in Japanese Laid-Open Patent Publications S58-25420.
Among conventional steels (a) to (i), the steel (b), which is free from coarse carbide was developed in consideration that coarse carbides trigger SSC.
A steel free from such coarse carbides can be produced by providing a quenching and a short time tempering treatment to a low alloy steel, which is designed to include chromium and other various elements in order to prevent coarse carbides from leaving, precipitating or growing, during the heat treatment.
In general, a steel which needs SSC resistance, is quenched to obtain a martensitic microstructure in which carbon exists in solution state, and thereafter tempered to allow precipitation of fine carbides. For this purpose, a low alloy steel which contains chromium so as to increase a hardenability of steel is usually used as a base steel.
In case of tempering treatment being exerted at a relatively low temperature, carbides precipitate in a film like state on prior austenitic grain boundaries. To prevent this, it is practiced that a suitable amount of molybdenum is added to a low alloy steel, which is tempered at a relatively high temperature.
Precipitated carbides grow to be coarse if the tempering process continues for a longer time, and therefore an induction heating is applied to shorten a tempering time.
Further, since carbides precipitate on grain boundaries and tend to grow, in order to achieve dispersion of carbides, the microstructure is made into fine grain by various methods.
As carbides which precipitate in a low alloy steel containing chromium and molybdenum, several types of carbide, such as M.sub.3 C type, M.sub.7 C.sub.3 type and M.sub.23 C.sub.6 type, are conventionally known. Among them, M.sub.23 C.sub.6 type carbide is liable to become coarse. In view of thermodynamics, those are more stable in due order of M.sub.3 C type, M.sub.7 C.sub.3 type and M.sub.23 C.sub.6 type, and therefore coarse carbides of M.sub.23 C.sub.6 type unavoidably precipitate in a quenched and tempered steel containing chromium and molybdenum. In case of an extremely large amount of molybdenum content, M.sub.2 C type also precipitates. Since M.sub.2 C type carbide is of a needle like shape and has a high stress concentration factor, it reduces SSC resistance.
Where, M represents metal, and means metallic elements such as iron, chromium, molybdenum, vanadium, etc. Specifically, M.sub.3 C and M.sub.23 C can be, for instance, Fe.sub.3 C, Cr.sub.23 C.sub.6 and so on.
As for a precipitation control method of M.sub.23 C.sub.6 type coarse carbide, a short time tempering process is most effective. As described above, this short time tempering process has been conventionally used. However, this short time tempering process essentially requires an induction heating facility, therefore requires a substantially large amount of capital investment.
Grain refining is also effective to control precipitation of coarse carbides. In order to achieve sufficient grain refining, however, it is necessary to conduct a heat treatment twice or more, and/or to carry out a quenching treatment at a lower temperature. As a result, not only the heat treatment costs increase, but also the amount of solution of alloy elements reduces, eventually necessitating an increase in the amount of alloy elements added, which results in increasing material costs. Further, since grain refining inevitably causes the deterioration of the hardenability, a rapid cooling is essential in order to obtain a martensitic microstructure. Therefore, special cooling equipments are required, thereby requiring a substantially large amount of capital investment.
There are various reports about how alloy elements affect a precipitation behavior of carbides and a SSC resistance in a chromium-molybdenum steel. For example, Metallurgical Transactions A. 16A, May 1985, p.935 "Sulfide Stress Cracking of High Strength Modified Cr-Mo Steels" reports that an addition of about 0.1% vanadium is effective to improve SSC resistance. The steel reported here, however, fails to achieve a desired degree of SSC resistance in NACE TM0177 solution under a condition of applying 85% stress of the specified minimum yield stress (SMYS).